National Physical Laboratory

UV cleaning of platinum-iridium mass standards

UV/Ozone Mass Cleaning
Figure 1: UV/ozone
mass cleaning

Cleaning of platinum-iridium mass standards using ultra violet light and ozone provides a non-contact alternative to solvent/steam cleaning.

NPL scientists have developed an apparatus for cleaning mass standards that uses a combination of ozone and ultra violet light (Figure 1). UV light of wavelength 185 nm creates the ozone which bonds with the hydrocarbon contamination on the surface of the mass. A second UV wavelength at 250 nm breaks these carbon-oxygen bonds, releasing the contamination from the surface in the form of CO2 (P J Cumpson and M P Seah, Metrologia 33 (1996) 507).

Mass Steam Cleaning
Figure 2: Mass
steam cleaning

Cleaning of a platinum-iridium kilogram mass standard is necessary as contaminant build up on its surface can typically be 3 µg per year. The current cleaning method involves rubbing each standard with chamois leather soaked in solvent followed by removal of the solvent with a jet of steam (Figure 2). As the current method is operator dependent, it is difficult to reproduce and can introduce instability in a mass standard.

NPL's UV/ozone technique has been evaluated by exposing Platinum foil that had gained contamination over three years to UV light over a range of intensities and duration. X-ray photoelectron spectroscopy (XPS) was used to measure the amount of contamination removed from the foil's surface. The results give an optimum UV intensity of around 12 mWcm-2 and exposure time of 1 to 1.5 hours (Figure 3). This exposure removed about 5 µg of contamination from the foil's surface (equivalent to a contamination thickness of 2 nm).

UV Dirt Removal
Figure 3: UV dirt removal

Further XPS measurements were made on a platinum foil cleaned in boiling water to compare the effectiveness of the technique with conventional cleaning (Figure 4). Approximately 3.5 µg of contamination was removed with this cleaning process.

Boiling Water Dirt Removal
Figure 4: Boiling water dirt removal

Following successful trials with platinum foil, the UV/ozone cleaning equipment has been modified to enable cleaning of Platinum-Iridium (Pt-Ir) mass standards (Figure 5). A Pt-Ir mass standard with surface hydrocarbon contamination will be cleaned to confirm the validity of the technique.

Mass
Figure 5: Platinum-Iridium (Pt-Ir) mass

Last Updated: 17 Dec 2012
Created: 10 Sep 2007

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